The object of the invention is a method of preventing the formation of dangerous underpressure in a respiratory system.
When the patient is connected to a respirator, a pressure generally prevails which is at least the same as the ambient pressure. During inspiration an overpressure is required to fill the patient's lungs, and the pressure decrease is often even restricted in the expiration phase in order to prevent the collapsing of the patient's lungs, for which is used the term PEEP, Positive End-Expiratory Pressure. The occurrence of underpressure, on the other hand, is exceptional in such a system and is generally considered as injurious. At the worst underpressure can cause absorption of fluid from the tissue to the lungs and lead to permanent injuries. The allowed underpressure is only some centimeters of water.
In connection with respiratory breathing it is nowadays customary to measure the concentrations of gases inspired and expired, especially the carbon dioxide, oxygen and anesthetic gas concentrations. This is generally made by aspirating a small amount of gas, typically about 200 ml/min into a monitor that analyzes the concentrations of the desired gases and displays the result on e.g. a display.
The amount of sample gas taken by the gas monitor is in normal conditions insignificant compared to the respiratory gas amount fed into the system, the so called fresh gas flow, which generally is several liters per minute. Even in extreme cases, when the fresh gas flow is below 1 l/min., when a so called low flow anesthesia is involved, the gas amount discharged from the system can easily be substituted by increasing the fresh gas flow correspondingly.
There are, however, cases when due to human error or equipment failure the gas feed into the system has been interrupted. The gas amount aspirated by the monitor is in such cases not substituted at all, but the monitor aspirates into the respiratory system a growing underpressure, which is limited only by the capability of the pump to generate underpressure, and on the other hand by a possible leakage in the system, which again in itself is not a desired property. The underpressure can, dependent on the gas volume of the system, fairly quickly reach a limit where the patient's safety for the above reasons is jeopardized.
There has been no previous method available to secure the prevention of such an underpressure formation. The gas monitors generally measure the pressure of their own internal sampling system, which in principle is also dependent on the pressure of the patient system. The dependence is, however, not unambiguous, as the difference between these pressures also depends e.g. on the flow resistance in the sampling line. A sudden pressure decrease is generally caused by constriction in the sampling line, wherefore it cannot reliably be used to indicate the pressure prevailing in the patient tube system.
New respirators have usually a built-in airway pressure monitoring, some also provided to alarm when the pressure falls below a preset limit. There are, however, quite many old devices in use which lack these properties. Moreover, it is quite possible that despite monitoring and alarm, the locating of the problem and the repair of the failure take so long that the patient will already suffer from the situation. The ventilator patient tube might also constrict, thus putting the ventilator safety equipment out of function. No means have been provided so far to prevent the occurrence of such a situation.